I read a lot of motivational-type books. In fact, I read them over and over again. Two of my favorites are the classic, "Think and Grow Rich" by Napoleon Hill and the more contemporary "The Secret" by Rhonda Byrne. "Think and Grow Rich" has to do with thoughts being things and how your thoughts and beliefs determine what actually happens to you. The theme of "The Secret," in my opinion, is to focus on what you want, not on what you don't want. I read these books over and over. "Focus on what you want" has become my guide.
In my own experience, sometimes I've taken on too many projects at once, juggling between many and not putting 100% of my attention to one thing. I've become much better at sorting these things out, prioritizing what needs to be done first and focusing on the one project I'm working on. I try to be organized, make a to-do list for myself every day, get assignments done in advance so that I don't have to be in a panic to produce gags at the last minute. This works for me.
I guess you have to constantly ask yourself what's the most important thing you need to be working on at that moment. Once you identify what your priority is you have to focus on it. If you're working on your own strip, or someone else's strip, or non-syndicated single-panel gag cartoons for a specific publication, you have to live and breath it; make it a big part of your life, look at it from all angles, put all your energy into it and believe that you will succeed at it.
Focus on what you want - not on what you don't want.
And, don't work completely in isolation. Get out and talk to people in the same field if at all possible, or correspond with people by e-mail or phone. Show people what you do. Send your work out to the world. Be proud of it. Here's another one of my favorite sayings: "You won't catch fish if you don't go fishing."
Finally: Enjoy what you do.